Polymers are basically heterogeneous materials, heterogeneity being exhibited through a variety of ways such as distribution of chain lengths, differences in chemical composition from chain to chain, and through the architecture of the chain as in branched and crosslinked structures. Each form of heterogeneity exerts an influence on the behavior and the properties of the ultimate product.
For semi-crystalline resins such as polyethylene, it is branching which is of particular interest from the point of view of the impact on resin properties and use. Branching, which may be characterized as long chain branching (LCB) or short chain branching (SCB), can arise through chain transfer reactions during free radical polymerization at high pressure or by copolymerization with other comonomers. It is now known that the overall branching level influences resin properties through its control o crystallinity and morphology. Each branch point in a chain disrupts the local order during crystallization and reduces the degree of crystallinity. Thus, there is a direct relationship between crystallinity and the degree of branching. However, while size exclusion chromatography (SEC) provides insight into long chain branching characteristics of crystalline polymers (Lecacheux et al. (I), J. Ap. Poly. Sci., 27:4687-4877 (1982); Lecacheux et al. (II), J. Ap. Poly. Sci., 29:1569-1579 (1984)), size exclusion chromatography leaves much to be desired where short chain branching information is required.
Temperature rising elution fractionation (TREF) is a technique of fractionating a polymer according to crystallinity and has particular utility for evaluating short chain branching. The technique of TREF was first used by Desreux et al., Bull. Soc. Chim. Belg., 59:476 (1950), wherein a 5 gm sample of polyethylene was deposited from toluene solution onto granules and packed into a column. Elution was carried out by flowing toluene through the column as temperature was increased TREF has been employed as a means of studying short chain branched distribution in high pressure, low density polyethylene (LDPE). See Shirayama, K. et al., Jour Poly Sci.-A, 3:907 (1955) and Bergstrom, C. et al. Kemia-Kemi Helsinki, 23:1, 47 (1976). TREF has been used as well as a means of studying short chain branch distribution in ethylenepropylene copolymers (Solda, S. et al., Kobunshi Kagaku (Eng. Ed.), 2, No. 10:866 (1973)) and ethylene-butene copolymers (Wilde, L. et al., (I), J. Poly. Sci. Poly. Phys. Ed., 20:441 (1982); Wilde, L. et al., (II), Poly. Preprints, 18(2):182 (1977)); and Wilde, L. et al., (III), Poly. Preprints, 23(2):133 (1982)).
From an operative standpoint, TREF (also known as preparative TREF) has certain disadvantages. One is the time required to perform a fractionation, which typically is about seven days. The time-consuming step is the separation of polymers from each of approximately fourteen fractions so that the mass per fraction may be obtained and for further analysis such as determining short chain branching by infrared (IR) analysis. The second deficiency of the preparative procedure involves the manner in which the short chain branching distribution curve is constructed. Due to the limited number of data points available (approximately fourteen), the weight percent distribution profile is not well defined. Also, mass measurements of fractions containing small amounts of polymer are subject to rather large errors because of the difficulty in removing all polymers from the liquid phase during the workup procedure. Further, each of the fractions must be filtered, dried, weighed and pressed into films for analysis. The efficiency of the separation process is less than ideal owing to the large size of the columns, the physical dimensions of the system making it difficult to avoid channeling, "dead" spots, and temperature gradients along the length and across the diameter of the column.
An improvement upon the TREF system, although still requiring the use of preparative TREF for calibration purposes, was the development of analytical temperature rising elution fractionation (ATREF), reported upon by Wilde, L. et al. (I) supra. ATREF overcomes the limitations of preparative TREF by reducing the column (and sample) size considerably and continuously monitoring the amount of polymer eluted as a function of temperature using an in-line detector such as a differential refractometer. On-stream monitoring of polymer concentration at each column elution temperature obviates the need to physically segregate and weigh each fraction collected.
However, branching distribution alone is not sufficient to provide a evaluation of overall resin structure. Wilde, L. et al. (I), supra, proposed to utilize data resulting from an ATREF type analysis in conjunction with separate data produced by size exclusion chromatography to provide an evaluation of overall resin structure. Aside from the complexity of coupling these separate analyses, such an instrument would hardly be a candidate for routine analysis in light of the inherent maintenance requirements for keeping such a device operational. And, while this is an improvement over the preparative method, the data is still essentially a disjointed set of separate analyses over a TREF run.
Thus, prior to the present invention, analytical temperature rising elution fractionation and high temperature continuous viscometry have not been utilized as a means for evaluating polymers.